The goals of the PDT Physics core are to: 1) Provide Laser and other light source support for all projects; 2) Perform high-quality and reliable in vivo light dosimetry for projects (1,3,4 and 5); 3) Provide absolute light dosimetry standards for all projects; 4) Provide explicit PDT dosimetry for tissue optical properties, drug concentrations, and tissue oxygenation before, after, and during PDT. The physics core is responsible for calibrating the absolute output power of laser sources, ensuring that they are traceable to standards maintained by the National Institute of Science and Technology (NIST). The core performs in vivo light dosimetry to achieve accurate light fluence measurement in PDT treatments, such as the Intraperitoneal and Pleural PDT studies. Laser equipment operating at treatment wavelengths (532-730 nm) for PDT are maintained, along with light sources covering wavelengths between 400 - 900 nm for absorption and fluorescence spectroscopy. The physics core translates the spectroscopic techniques developed in the physics project for in vivo PDT dosimetry into clinical trials. The physics core ensures laser safety, maintains quality assurance of physics equipment, and works to characterize light fluence distributions in tissue. It helps to implement useful technologies developed in the physics project (Project 4) to routine clinical applications.